Full-width under-rail tool box, full of jacks, tools, extra power cord, etc.
Usually about 200 lbs of firewood.
Propane tanks and cookstove.
Extra spare for the truck.
One bicycle (other in the fifth-wheel basement)
Ramps and blocks for leveling fifth.
Wheel chocks.
Spare carpet for in front of camper door.
And anything else I can cram in the corners!

I have a 50 gallon aux. fuel tank, a 50 gallon fresh water tank, and 3 five gallon gas tanks in front of the 5th wheel. Behind the 5th wheel is the 35 gallon sewer tote with wheel chocks. I have an 8t. bed on a 1 ton dually. I have a roll up, lockable tonneau cover that is used when the 5er isn't attached. The 3 gas tanks and the tote are secured with cables that lock in case they decided to go for a walk. The gas tanks and the tote are also held in place with wratchit tie downs.